This invention relates to an axially deflectable rotary transducer head assembly for a video cassette tape recorder (VCR).
Automatic VCR tracking systems in which the magnetic transducer head is controllably deflected or adjusted in the direction of the tape width to ensure the accurate tracing of data tracks, recorded on the tape at a normal feed speed, during playback or reproduction at a different speed are known.
One such system is disclosed in commonly assigned, copending U.S. Pat. No. 4,882,635 filed on July 14, 1987 as illustrated in the cross-sectional and plan views of FIGS. 1 and 2, wherein reference numeral 1 designates a rotary shaft, 2 is a fixed lower drum, 3 is a rotary upper drum, 6 are bearings for the shaft, 7 is an upper transmitter which rotates with the drum 3, 8 is a lower transmitter mounted on the fixed drum 2, 9 is a base block for mounting the upper drum to the shaft, and 13 designates a magnetic tape wrapped obliquely around the outer surfaces of the upper and lower drums.
A drive unit 4a disposed in a recess 50 of the upper drum 3 and secured thereto by a screw 48 mounts a magnetic transducer head 5 for controlled vertical displacement pursuant to automatic tracking speed compensation, i.e. displacement in a direction parallel to that of the drum shaft 1. A plurality of electrodes 15 mounted to the base block 9 are disposed in sliding engagement with an equal plurality of fixed contacts 14. A connector post 16 is electrically coupled to the drive unit 4a through the electrodes 15, a connector 12, and a wiring plate 11. The head 5 is electrically connected to the upper transmitter 7 through a connector post 10, the wiring plate 11, and the connector 12.
The drive unit 4a will be described in greater detail with reference to FIGS. 3 through 6, wherein reference numeral 41 designates a circular lower spring plate made of a non-magnetic material and having an outwardly extending arm or tab on which the head 5 is mounted, 43 is a coil bobbin attached to the spring plate 41, 44 is a deflection control coil wound around the bobbin, 42 is a circular upper spring plate attached to the bobbin and deformable together with the lower spring plate when the bobbin is linearly driven in a vertical direction, 45 and 46 are lower and upper, cylindrical, permanent pole magnets extending within the bobbin and having oppositely oriented polarities, 46a designates a ferromagnetic center pole disposed within the bobbin between the two permanent magnets, and 47 is a yoke assembly constituted by lower, intermediate and upper portions 47a, 47b and 47c. A window 49 is provided in the lower portion of the assembly, through which the tab of the lower spring plate extends such that the head 5 fixed to the end thereof is disposed in sliding contact with the tape 13 as shown in FIG. 1. The bobbin 43 and coil 44 are disposed for vertical movement in an annular gap defined between the intermediate yoke portion 47b and the magnets 45, 46 and the center pole 46a. The outer peripheries of the spring plates 41, 42 are clamped between the yoke assembly portions as shown in FIG. 4. A threaded bore 48a in the upper portion 47c accommodates the screw 48 for mounting the yoke assembly 47 to the upper wall 50a of the upper drum recess 50.
Referring to FIG. 4, a first closed magnetic circuit is formed by the yoke portions 47a, 47b and the permanent magnet 45 through which a magnetic flux D flows radially outwardly from the center pole 46a in a toroidal path. A second closed magnetic circuit is similarly formed by the yoke portions 47b, 47c and the permanent magnet 46 to accommodate a magnetic flux E which flows in a toroidal direction opposite to that of flux D. The total flux traversing the coil 44 is thus the sum of the two fluxes D and E. Accordingly, automatic tracking compensation may be implemented by applying an appropriate control signal to the coil 44 from a source, not shown, which serves to linearly displace the coil and thus the bobbin, the spring plates 41, 42 and the magnetic head 5 in a vertical direction parallel to the drum shaft 1.
A problem with such a tracking compensation arrangement arises due to the plates 41, 42 having substantially the same spring constant. As a result, when the energized coil 44 and the bobbin 43 undergo an axially elongating thermal expansion, the upper and lower spring plates are bent or distorted by equal amounts to accommodate such expansion as shown in FIG. 6. As a result the vertical position of the magnetic head 5 relative to the drum and thus the tape width, shown in FIG. 6 with reference to the wall 50a of the upper drum recess, increases from F to G which adversely affects the head tracking accuracy and thus the fidelity of the reproduced signal. Such distortion might be at least partially reduced by fabricating the coil bobbin 43 from a material having a very low coefficient of thermal expansion, but such materials are rather "exotic" and thus expensive.